The present invention relates to oil well drilling cements, and more particularly to an additive for an oil well drilling cement having dispersant effectiveness and low set time retardation.
Oil well drilling cement compositions are typically used for sealing or cementing subterranean zones such as the annular space in an oil well between the surrounding formation and casing. In cementing the annular space of an oil well, a cement slurry is pumped down the inside of the casing back up the outside of the casing through the annular space and then allowed to set.
A satisfactory cementing composition must have various desired functions which contribute to the success of a well drilling cementing operation. In order to accomplish these various functions, it has been found necessary to incorporate certain additives in a cementing composition to obtain a desired set of rheological properties for the cementing composition. For example, high temperatures are frequently encountered in subterranean zones and such temperatures will cause premature setting of the cementing composition. Thus, additives which extend or retard the setting time of the cement slurry are well known in the art so that there is adequate pumping time to move the aqueous cement slurry to the desired location. Additionally, dispersing agents are employed for promoting the formation and stabilization of the cement slurry. Typically, other additives such as defoaming agents, friction reducing agents and other conventional additives will be added to the cement composition.
Modified lignosulfonates derived from spent sulfite liquor obtained from the pulping of woods are known to be effective additives, both as a retardant or a dispersant, for obtaining the desired properties in cementing compositions. However, previously known dispersing agents made from modified lignosulfonates, called "thinners" since they function to reduce the effective viscosity of the cementing composition under drilling conditions, frequently also cause an increase in the retardation or setting time of the cement slurry. Therefore, it is desirable to provide a modified lignosulfonate for use as an oil well drilling cement additive that functions to "thin" cement slurries without significantly retarding the cement set time.